In the case of fork-lift trucks, loads of load forks are often lowered in a proportional manner by poppet valves having a constant opening behavior. The poppet valves in this case can be directly actuated or can be pilot-operated. The use of poppet valves is necessary, in particular, because fork-lift truck manufacturers require a “stacker-tight” load behavior allowing for only very minimal leakage. By preventing leakage in the hydraulic circuit, the load fork cannot lower on its own with or without a load, which would pose a safety problem.
In the known solutions, a load-independent limitation of maximum volumetric flow is carried out by a constant volumetric flow controller connected in series to the other components of the hydraulic circuit. The requirement that a maximum lowering speed of a load fork cannot be exceeded, independently of the prevailing load pressure, is met in this manner. The known control devices associated therewith have the disadvantage that, when the load forks are lowered without a load, only the dead weight of the moving parts, in particular in the form of the load forks, is available at the lift mast for pumping the hydraulic fluid from the working cylinder (plunger cylinder) back to the tank as a component of the hydraulic circuit. In certain designs of lift masts, the pressure at the cylinder can fall to values <10 bar. The greater the number of components through which fluid must flow during lowering, the lesser is the volumetric flow. In this connection, the known constant volumetric flow controller poses a particularly difficult obstacle to overcome, because the supply aperture thereof must be selected such that it is small enough that a control pressure differential of at least 7 bar can be established. Lesser control pressures would result in an unstable behavior in the hydraulic circuit of the working device and cannot be accepted for reasons of safety. Moreover, small regulating pressure differentials also must not result in noticeable impairments of lowering speeds. The use of a maximum volumetric flow limiter also has the disadvantage that, when transitioning from the maximum volumetric flow limitation back to the proportional characteristic curve, a disadvantageous rebound occurs. The rebound is manifested as a discontinuous movement of a load fork, which movement can pose another safety problem. Finally, a flow controller that is connected in series causes a not inconsiderable loss in pressure, which greatly slows the lowering of the load.
In addition, customers require a decreasing characteristic curve for the volumetric flow controller. For reasons of safety and practicality, the lowering speed must remain the same or slow down as the load increases. If this feature is carried out with the known control devices using the constant volumetric flow controller, this circumstance also results, in principle, in increased instability within the hydraulic circuit.
As an alternative to the solution having a maximum volumetric flow controller that is connected in series, the applicant has disclosed, in DE 101 02 409 A1, a control device in which a proportional directional poppet valve and a pressure regulator are connected in series in a hydraulic circuit. The users of these solutions now further require a valve that is even more cost-effective and requires less installation space.